(a) Field of the Invention
This invention relates to a process for the manufacture of 4,4'-dihydroxydiphenyl (biphenol, diphenol) and more particularly relates to a process for the purification of 4,4'-dihydroxydiphenyl which contains 4-monohydroxydiphenyl impurity.
(b) History of the Prior Art
Biphenol is useful as a reactant in condensation polymerizations and is particularly useful in forming phenolic type resins having good temperature resistance.
Biphenol was formed in the prior art by various methods. For example, halogenated diphenyl could be hydrolyzed (U.S. Pat. No. 3,413,341) or diphenyl could possibly be directly hydroxylated with peroxide in the presence of a suitable catalyst (U.S. Pat. No. 3,453,332).
One of the most suitable methods for the preparation of biphenol is sulfonation of diphenyl to form diphenyldisulfonic acid which is then reacted with an alkali metal hydroxide or alkali metal salt to form a diphenyldisulfonic acid salt followed by fusion of the diphenyldisulfonic acid salt with an alkali metal hydroxide to form the sodium salt of biphenol followed by dissolving of the reaction mass and acidification to form biphenol. Such processes are described in U.S. Pat. No. 2,368,361 and copending Patent Application Ser. No. 071,572 filed Aug. 31, 1979.
In all of the prior art processes and especially in those involving fusion reaction, a significant quantity, i.e., in excess of 1%, of 4-monohydroxydiphenyl is formed. In fusion reactions, this is due to the presence of diphenylmonosulfonic acid salt which is formed during sulfonation of diphenyl and which is exceedingly difficult to remove from the desired diphenyl disulfonic acid salt. Such impurities in hydrolysis of halogenated diphenyl result from the presence of monohalogenated diphenyl products which occur during the halogenation process and which are also difficult to remove from the dihalodiphenyl compounds. Similarly, direct hydroxylation with peroxide can result in the attachment of a single OH group which will result in undesired monhydroxy impurity.
Such monohydroxydiphenyl impurity is especially significant when biphenol is formed from fusion type reactions. In the past, purification methods usually employed to purify the biphenol product have been ineffective to remove monohydroxydiphenyl. Recrystallization is ineffective since the recrystallized product also contains recrystallized monohydroxydiphenyl. Distillation is ineffective due to the close boiling temperatures of monohydroxy and dihydroxydiphenyl and sublimation is ineffective since the monohydroxydiphenyl sublimes at a temperature close to the sublimation temperature of diphyroxydiphenyl and if anything, results in an increased concentration of monohydroxydiphenyl impurities since during sublimation, there is a higher concentration of monohydroxydiphenyl during the first part of the sublimation process. The presence of monohydroxydiphenyl in the 4,4'-dihydroxydiphenyl product is believed to be undesirable since when the 4,4'-dihydroxydiphenyl is used in a polymerization, it is believed that the monohydroxydiphenyl acts a chain terminator thus preventing the formation of polymers with molecular weights which are as high as would be obtainable in the absence of the monohydroxydiphenyl impurity.
It had been previously thought that the removal of the monohydroxydiphenyl impurity was much too costly since no inexpensive standard purification means appeared to efficiently accomplish the purification.